Mechanical Properties of Magnesia Single Crystals Corn pression

Abstract

Stress-strain curves of single crystals of magnesia compressed in the [IOO] direction are reported at temperatures from -196' to 1200°C.; curves are also shown for different rates of loading at room temperature. The crystals show considerable ductility at all temperatures and at room temperature can be deformed plastically about 6% before fracture at stresses which are about one-quarter of reported polycrystalline fracture strengths. The macroscopic yield drops apparently exponentially from an extrapolated value of 50,000 lb. per sq. in. at absolute zero to about 4500 lb. per sq. in. at temperatures of 900°C. and higher. Heat-treatment has an appreciable effect on the yield stress. The resistance of the material to deformation increases with the number of slip systems and bands activated because of the barriers to dislocation movements which occur at slip band intersections. At about 2 to 3% strain, stress concentrations begin to be relieved by small internal cracks which are not easily propagated. This effect is extensive before final macroscopic failure of the crystal occurs. Preliminary creep tests above the macroscopic yield stress and in the temperature range 800' to IOOO'C. show large instantaneous plastic deformations followed by slow constant-rate creep.

Dislocations and Plastic Deformation in MgO Crystals: A Review

“…Hulse and Pask have shown that MgO single crystal can deform plastically down to very low temperatures, measuring a fracture strain of about 6% at 77 K [14]. Surface analyses emphasize 90 • shear bands that confirm the joint activity of two conjugate 1 ⁄2<110>{110} slip systems as already observed in Day and Stokes' work [7].…”

“…Strain-hardening rates were also computed and show a satisfactory agreement with experiments [8,14,44] at 1000 K and 2000 K for both slip modes. Again, no multi-mode configurations between slip modes were investigated.…”

“…Several studies refer to the influence of point defects on CRSS in MgO [8,14,18,20,21]. Generally, the CRSS increase in the intermediate range of temperature is justified by the impurities oxidation state [21,31,32] and their spatial distribution [1,33,34], both parameters being intrinsically linked to the heat treatment processed before deformation.…”

“…Hulse, Copley and Pask performed a set of compression tests on single crystals investigating the CRSS and deformation processes in both slip modes [5,8,14]. Temperatures investigated range from 100 K up to 1400 K for the 1 ⁄2<110>{110} slip systems.…”

Dielectric breakdown of single crystal strontium titanate

“…No stress-strain curves have been taken at lower temperatures, but the brittle-to-ductile transitions in ionic crystals are usually fairly gradual. 14,15 In addition, local plastic flow may occur even if the material does not exhibit macroscopic yield, as evidenced by recent work on germanium. 16 Thus it is reasonable to suppose that plastic flow accounts for the observed thickness dependence and scatter in electric strength at −80°C and that it would cause an even larger effect at −195°C except that fracture occurs first.…”